A society where I am free to own you as a slave is not more free than a society that does not allow me to own you as a slave.
Owning slaves is a restriction on the freedom of others (the slaves). It reduces the sum total of human freedom in society. Yet how can this be if preventing it removes my freedom to own slaves, thereby also apparently reducing the sum total of freedom in society?
In fact preventing me from owning slaves does not restrict my freedom. It simply removes a means to an end, a demonstrably immoral means that also prevents others from pursuing their own ends (the slaves).
Banning slavery within society therefore increases the sum total of freedom within society. This can be explained in any number of ways, but the practical effects are clear.
A naive robust individualist might object that society is just an abstraction and it’s every man for himself. But society consists of individuals, robust or otherwise, and in saying that the sum total of human freedom in society is reduced we are simply saying that the probability of an individual’s freedom suffering is increased.
With the abolition of slavery in the West, Indentured Labour became popular. The intent of abolition was to free slaves, the effect of indentured labour was to remove the freedom of individuals. This ironised abolition. The forms of slavery and indentured labour might be different but their content and practical effects are similar, and deliberately so. Supporting human freedom meant opposing both despite one being legal and the other economic.
I mention this example because of two errors common made in discussing Free Software. (It is an extreme example, and if anyone has one less likely to offend that makes the same point I will gladly use it instead.)
The first is the claim that preventing people from removing the freedom of others as an end in itself is a reduction in freedom. This argument is usually made those who recommend the BSD licence for ethical rather than practical reasons. Indeed it is often the only “freedom” that is seen as worth defending. The flaws in this argument are obvious from the example above. Preventing people from removing the freedom of others is not an infraction on anyone’s freedom and will not prevent them from pursuing their ends within society.
The second is the claim that DRM or Tivoisation are technology rather than law and that Free Software licences have no business defending users against them. This argument is usually made by people who really should know better. The flaws in this argument are also obvious from our example. Free Software licences defend against the content of restrictions on freedom, not the form. To insist that a new form of restriction should not be defended against despite its effects is an aesthetic rather than an ethical argument.